1. Field of the Invention
The present invention relates to a technique of reinforcing olefinically unsaturated rubbers.
2. Description of the Background
The most important reinforcing fillers known to the present for olefinically unsaturated rubbers are active carbon blacks having a large surface area.
The replacement of carbon black by silicon-containing fillers of comparably large surface area is economically interesting for the preparation of pale or colored vulcanizates. Although the tear propagation strength of the vulcanizates prepared in this way is improved, all of the other rubber-technological properties are, however, significantly impaired. It is known to compensate for this shortcoming to a greater or lesser extent by adding a so-called coupling agent to the rubber. The coupling agent has the task of ensuring bonding between the hydrophilic silicon-containing filler and the hydrophobic unsaturated rubber components of the mixture. Particularly good coupling agents-for this purpose have proven to be organofunctional silanes, which form siloxane bonds with the silicon-containing filler by means of reactive silicon-containing groups and which form sulfur bonds with the rubber during the vulcanization by means of sulfur-containing groups (cf. Ullmann's Encyklopadie der Technischen Chemie, Volume 21, page 498, Verlag Chemie, 4th edition).
A known coupling agent is 3-mercaptopropyltriethoxysilane. However, its odor makes it use disadvantageous. For this reason, coupling agents which overcome this disadvantage have been employed which have the formula: EQU (RO).sub.3 Si--Y--Sx--Y--Si(OR).sub.3,
wherein R is methyl and/or ethyl, X is 2 to 6, usually about 4, and Y is a divalent saturated hydrocarbon radical having 1 to 5 carbon atoms and a linear or branched carbon chain, which is optionally interrupted by --O--, --S--, or --NH-- (German Auslegeschrift No. 2,712,866), or Y is ##STR2## (EP-Al-O,191,929).
Since coupling agents of the last-mentioned type contain an average of four sulfur atoms, they are known as "tetrasulfanes". However, these compounds exhibit the disadvantage that they must be employed, above all, in not inconsiderable amounts with respect to highly disperse silicas in order to obtain tension values, for example, at 300% elongation, which are improved by a multiple over a comparison vulcanizate which does not contain a coupling agent. In addition, the synthesis of a "tetrasulfane" requires anhydrous Na.sub.2 S.sub.4, whose preparation is not without problems because of the use of metallic sodium. A need therefore continues to exist for a coupling agent which improves upon the bonding between unsaturated rubber and added inorganic filler.